On the DNS Information page, find the "Authoritative Name Servers" box and copy down the information. (You'll find two server names of the format ns.xxx0.nearlyfreespeech.net.)

Go to your domain registrar's site and find the option to change name servers.

Replace whatever name server information they currently have with the information you found in the "Authoritative Name Servers" box.

Since each registrar has a slightly different way of editing name servers, you would need to contact them for specific help with using their system.

Once these steps are complete, you may have to wait awhile for the change to take effect. Changes in most top-level domains take less than 24 hours, and the major ones (.com, .net, and .org) often take effect within a few minutes.

If your domain registrar provides DNS service, you can also use that, although it is not recommended. Please see this FAQ entry for more information.